LxVMELLlBIlANCHIATA OF TUE LOWER MAKLS. 93 



Casts of this shell are usually found in collections marked G, rostellata, 

 and somewhat closely resemble those of that species, but they are propor- 

 tionally broader, with the beaks more nearly central; the valves are more 

 ventricose behind, and fuller along the basal line, with a decided sulcation 

 near the middle, on the right valve more marked than on the left, while on 

 the exterior the markings have been decidedly finer. It is extremely diffi- 

 cult to distinguish among the casts of this group of shells, as the characters 

 often change much according to the thickness of the substance, causing 

 them to lose or retain the form or surface-markings to a greater or less de- 

 gree. But the differences noted between this and its nearest allies will, I 

 think, serve to distinguish these readily. 



Formation and locality. — Conrad's type came from the Lower Green 

 Marls near Burlington, New Jersey, and I should conclude from the lith- 

 ological characters of tlie others which I Jiave seen that they are all from that 

 region, and probably all from the same bed. 



Cibota obesa, u. sp. 



Plate XI, Figs. 30 and 31. 



Shell small, with full and very ventricose valves, large tumid beaks 

 situated opposite the anterior third of the length, slightly enrolled, and 

 distant from each other as shown on the iaternal cast. Form of the outline 

 trapezoidal, the length of the cast nearly twice the height, exclusive of the 

 projection of the beaks; anterior end vertically rounded; posterior obliquely 

 truncate; extremity obtusely pointed: basal line full, but constricted just 

 anterior to the middle by the very marked but short and broad byssal 

 opening; area two-thirds the length of the valve and moderately wide. On 

 the casts the muscular imprints are very distinctly marked and of fair size, 

 no muscular ridge; the outer margin indicating a strong and abrupt thicken- 

 ing of the valves with a crenulated border; radiating hues indicating mod- 

 erately fine striae show on nearly all parts of the cast, but strongest on the 

 postero-basal section. 



The general form of this species is like a dwarfed and extremely ven- 

 tricose specimen of C, uniopsis Conrad, but is so perfectly neat and sym- 

 metrical in its shape as to preclude the idea of a stunted individual. The 

 valves are, however, equally ventricose, while those of that species usually 



